The prior art is replete with processes for the purification of diolefin feed streams by removal of acetylenic impurities. These processes are generally characterized as belonging to one of four major categories, namely, (a) hydrogenation, (b) oxidation, (c) polymerization, and, (d) cracking, each generally being regarded as unrelated to the others.
A simple, physical distinction between the processes may be noted, in that only polymerization processes in which the acetylenic impurities are polymerized, are practiced in the liquid phase. For example, a polymerization catalyst useful in the liquid polymerization of alkyne impurities is disclosed in U.S. Pat. No. 3,105,858. In contrast, cracking processes in particular, are vapor phase processes carried out by contacting the impure feedstream with a cracking catalyst at relatively high temperature. Another noteworthy distinction is that, in general, cracking catalysts, polymerization catalysts, and hydrogenation or oxidation catalysts are each specific for the particular category of process in which each is most efficiently used.
A notable exception is the hydrogenation catalyst disclosed in the cracking process of U.S. Pat. No. 4,174,355 (hereafter the '355 patent) as a highly effective cracking catalyst. The effectiveness of this bifunctional catalyst as a cracking catalyst is thought to be predicated, at least in part upon the unusual operating characteristics of such a process, though the precise mechanism of how a typical hydrogenation catalyst also performs as a cracking catalyst is not known. For example, a vapor phase process provides higher diffusivities of the reactants, which decreases the mass transfer resistance. Since the difference in diffusivities between the liquid and vapor phases is several orders of magnitude, the latter being higher, a much longer residence time in the liquid phase would be indicated. Since the temperature at which the liquid phase process is to be carried out is unavoidably relatively high for a temperature-sensitive diolefin monomer, the liquid phase is contraindicated.
In the category of polymerization processes carried out in the liquid phase, the operating temperature is necessarily relatively low, usually lower than 150.degree. F., yet the losses due to polymerization of the monomer being purified are still so high that such known processes are only of academic interest.
One of the earliest attempts to catalytically purify conjugated diolefins contaminated with acetylenic hydrocarbons is documented in U.S. Pat. No. 2,398,301 (4/1946) to Frevel, L. K. Soon thereafter, another catalytic process, stated to be a selective hydration process, was disclosed in U.S. Pat. No. 2,408,970 to Doumani et al for the removal of acetylenic impurities in hydrocarbon mixtures containing butadiene. At present, minor quantities of IPEA, and other C.sub.2 -C.sub.5 acetylene are removed by selective hydrogenation of the acetylenes over a supported copper catalyst. Such processes are taught in U.S. Pat. No. 3,076,858 to Frevel et al; in U.S. Pat. No. 3,634,536 to Frevel L. K. and Dressley, L. J.; and, 3,751,508 to Fujiso et al, inter alia. Not long thereafter, an adsorption process carried out in the temperature range of 25.degree. C.-175.degree. C. was disclosed in U.S. Pat. No. 3,754,050 to Duyverman et al. Still more recently, U.S. Pat. No. 3,897,511 to Frevel, L. K. and Dressley, L. J., disclosed a catalytic process for removal of alpha-acetylenic impurities by their adsorption on a supported catalyst consisting essentially of a mixture of finely divided copper metal and a minor proportion of at least one polyvalent activator metal.
Polymer grade butadiene for the cis-polybutadiene polymerization system is obtained by purification of butadiene containing unacceptably high levels of apha-acetylenes such as vinyl acetylene and methyl acetylene. U.S. Pat. No. 3,897,511 teaches the selective chemisorption of alpha-acetylenes on copper catalysts activated with NiO, CoO, CrO or MnO. The activated copper catalyst is reduced with hydrogen prior to use. British Pat. No. 1,291,397 teaches a mixed CuO/Zno catalyst which can also be used for chemisorption of alpha-acetylenes.
The prior art is replete with a multiplicity of hydrogenation catalyst particularly suited for hydrogenation of acetylenic and other impurities in conjugated diolefin streams. A few of these catalysts are said to effectively lower the final acetylenic concentration of a feedstream from 1 percent to below 100 ppm without excessive conversion of the diene to a monoolefin or alkane, but the period of time over which the activity can be maintained is quite unpredictable. For one reason or another, some hydrogenation catalysts make for more successful hydrogenation processes than others, and the search for economically competitive processes, whether by hydrogenation or not, continues unremittingly.
As for a process other than selective hydrogenation for removal of acetylenic impurities from crude butadiene or isoprene, it will be evident that chemisorption of the impurities on active sites, for later removal of the impurities, necessitates impractically large quantities of adsorbent, even if the adsorbent has high surface area. Though neither selective hydrogenation nor chemisorption-desorption is as energy intensive as a vapor phase catalytic cracking process, none of the foregoing is economically as attractive as an energy non-intensive liquid phase process.
Relatively little interest has been directed to the conversion of alkynes by contact with base metal oxide catalysts without hydrogenation or hydration of the alkynes. Catalysts consisting of finely divided copper alone or mixed with an activator metal were known to be useful for removal of the alkynes by selectively decomposing or polymerizing these contaminants, but such a process, inter alia, was known to be subject to one or more disadvantages, as specifically stated in aforementioned U.S. Pat. No. 3,897,511, column 1, lines 26-43. As stated in the earlier Frevel U.S. Pat. No. 2,398,301, temperatures above 200.degree. C. (392.degree. F.) were indicated, 275.degree. C. (527.degree. F.) to 325.degree. C. (617.degree. F.) being preferred (page 2, right hand column, line 38). At these relatively higher temperatures, higher than 200.degree.-260.degree. F., not only alpha-acetylenes but also cyclopentadiene (hereafter "CPD" for brevity), is removed, and unavoidably, as evidenced by the exothermic reaction noted, a sufficiently large proportion of desirable diolefins are converted to mask the endothermic cracking of acetylenic impurities.
From a practical point of view, it is economically undesirable to hydrogenate a large feedstream of crude diolefin, no matter how selectively the hydrogenation can be effected, unless the hydrogenated feedstream is further distilled to produce an overhead with leas than 50 ppm alpha-acetylenes, and, the impurities-rich bottoms are recycled to the hydrogenation unit for further hydrogenation. As already indicated hereinabove, it is also economically undesirable to catalytically crack the impurities in a large feedstream because of the energy required to vaporize the feedstream. Stated differently, there are many economically viable options for producing a diolefin feedstream with 1000 ppm or more alpha-acetylenic impurities, but to purify the feedstream still further, one is pincered between the high costs of purification due to losses caused by hydrogenating desired diolefin along with the impurities, and, the high cost of vapor phase catalytic cracking, because of high regeneration costs of catalyst and shortened catalyst life. The '355 process is the only vapor phase catalytic cracking process known to me which can purify a diolefin feedstream having about 1000 ppm alpha-acetylenic impurities more economically than prior art processes. Nevertheless, the high cost of vaporizing the feedstream still was an economic burden. We are unaware of any prior art which teaches an energy non-intensive process for the conversion of acetylenic impurities and their removal by the use of a supported copper oxide or silver oxide bifunctional catalyst, in the absence of hydrogen, in a liquid phase, effectively inhibited, predominantly catalytic cracking process.